Proton concentration jumps and generation of transmembrane pH-gradients by photolysis of 4-formyl-6-methoxy-3-nitrophenoxyacetic acid

Influenza virus-membrane fusion triggered by proton uncaging for single particle studies of fusion kinetics

We report a method for studying membrane fusion, focusing on influenza virus fusion to lipid bilayers, which provides high temporal resolution through the rapid and coordinated initiation of individual virus fusion events. Each fusion event proceeds through a series of steps, much like multistep chemical reaction. Fusion is initiated by a rapid decrease in pH that accompanies the "uncaging" of an effector molecule from o-nitrobenzaldehyde, a photoisomerizable compound that releases a proton to the surrounding solution within microseconds of long-wave ultraviolet irradiation. In order to quantify pH values upon UV irradiation and uncaging, we introduce a simple silica nanoparticle pH sensor, useful for reporting the pH in homogeneous nanoliter volumes under conditions where traditional organic dye-type pH probes fail. Subsequent single-virion fusion events are monitored using total internal reflection fluorescence microscopy. Statistical analysis of these stochastic events uncovers kinetic information about the fusion reaction. This approach reveals that the kinetic parameters obtained from the data are sensitive to the rate at which protons are delivered to the bound viruses. Higher resolution measurements can enhance fundamental fusion studies and aid antiviral antifusogenic drug development.

Caged molecules: principles and practical considerations

A caged molecule is an inert but photosensitive molecule that is transformed by photolysis into a biologically active molecule at high speed (typically 1 msec). The process is referred to as photorelease. The spatial resolution of photorelease is limited by the properties of light; submicrometer resolution is potentially achievable. Therefore, focal photorelease of caged molecules enables one to control biological processes with high spatio-temporal precision. The principles underlying caged molecules as well as practical considerations for their use are discussed in this unit.

Sequence of late molecular events in the activation of rhodopsin

Activation of the G protein-coupled receptor rhodopsin involves both the motion of transmembrane helix 6 (TM6) and proton exchange events. To study how these activation steps relate to each other, spin-labeled rhodopsin in solutions of dodecyl maltoside was used so that time-resolved TM6 motion and proton exchange could each be monitored as a function of pH and temperature after an activating light flash. The results reveal that the motion of TM6 is not synchronized with deprotonation of the Schiff base that binds the chromophore to the protein but is an order of magnitude slower at 30 degrees C. However, TM6 motion and the uptake of a proton from solution in the neutral pH range follow the same time course. Importantly, the motion of TM6 is virtually independent of pH, as is Schiff base deprotonation under the conditions used, whereas proton uptake titrates with a pK of 6.5. This finding shows that proton uptake is a consequence rather than a cause of helix motion. Activated rhodopsin binds to and subsequently activates the cognate G protein, transducin. It has been shown that peptides derived from the C terminus of the transducin alpha-subunit mimic in part binding of the intact G protein. These peptides are found to bind to rhodopsin after TM6 movement, resulting in the release of protons. Collectively, the data suggest the following temporal sequence of events involved in activation: (i) internal Schiff base proton transfer; (ii) TM6 movement; and (iii) proton uptake from solution and binding of transducin.

Kinetics of proton binding to the sarcoplasmic reticulum Ca-ATPase in the E1 state

A new caged proton, 2-methoxy-5-nitrophenyl sulfate, was synthesized and used in time-resolved pH jump experiments to study proton binding in the sarcoplasmic reticulum Ca-ATPase. The major advantage of this compound is that it does not produce significant artifacts in experiments in which the fluorescent styryl dye 2BITC is used to monitor ion movements in the Ca pump. Two rate-limiting processes were resolved and their dependence on pH, Ca(2+) concentration, and temperature investigated. The faster process showed a relaxation time between 4 and 8 ms independent on pH and Ca(2+) concentration, and the time constant of the slower process varied between 31 ms (0 Ca(2+)) and 100 ms (100 microM Ca(2+)). A consistent mechanism to explain the results was derived in agreement with previous studies and the generally accepted Post-Albers scheme of the pump cycle. This mechanism requires that under physiological conditions the ion-binding sites are always occupied and two protons and a Ca(2+) ion replace each other. In the absence of ATP at low pH a nonphysiological state can be induced in which up to four protons bind to the Ca pump in the E(1) conformation. So far it could not be verified whether these additional protons bind to amino acid side chains or are coordinated as hydronium ions.

pH-Dependence of extrinsic and intrinsic H(+)-ion mobility in the rat ventricular myocyte, investigated using flash photolysis of a caged-H(+) compound

Passive H(+)-ion mobility within eukaryotic cells is low, due to H(+)-ion binding to cytoplasmic buffers. A localized intracellular acidosis can therefore persist for seconds or even minutes. Because H(+)-ions modulate so many biological processes, spatial intracellular pH (pH(i))-regulation becomes important for coordinating cellular activity. We have investigated spatial pH(i)-regulation in single and paired ventricular myocytes from rat heart by inducing a localized intracellular acid-load, while confocally imaging pH(i) using the pH-fluorophore, carboxy-SNARF-1. We present a novel method for localizing the acid-load. This involves the intracellular photolytic uncaging of H(+)-ions from a membrane-permeant acid-donor, 2-nitrobenzaldehyde. The subsequent spatial pH(i)-changes are consistent with intracellular H(+)-mobility and cell-to-cell H(+)-permeability constants measured using more conventional acid-loading techniques. We use the method to investigate the effect of reducing pH(i) on intrinsic (non-CO(2)/HCO(3)(-) buffer-dependent) and extrinsic (CO(2)/HCO(3)(-) buffer-dependent) components of H(i)(+)-mobility. We find that although both components mediate spatial regulation of pH within the cell, their ability to do so declines sharply at low pH(i). Thus acidosis severely slows intracellular H(+)-ion movement. This can result in spatial pH(i) nonuniformity, particularly during the stimulation of sarcolemmal Na(+)-H(+) exchange. Intracellular acidosis thus presents a window of vulnerability in the spatial coordination of cellular function.

Deprotonation yields, pKa, and aci-nitro decay rates in some substituted o-nitrobenzaldehydes

In this paper we report the deprotonation yields, the pKa, and decay kinetics of the aci-nitro intermediates of some substituted 2-nitrobenzaldehydes that can be used as photoactivatable caged proton compounds. The decay of the aci-nitro absorbance for 2-nitrobenzaldehyde occurs within a few nanoseconds from photoexcitation. Addition of electron donating methoxy substituents at positions 4 and 5 leads to lower deprotonation yields, higher pKa, and slower decays of the aci-nitro intermediates. On the contrary, the decay rate is accelerated by the introduction of an electron-withdrawing Cl atom at position 4 in the phenyl ring, with little influence on the deprotonation yield and pKa of the aci-nitro intermediate.

Characterization of a new caged proton capable of inducing large pH jumps

A new caged proton, 1-(2-nitrophenyl)ethyl sulfate (caged sulfate), is characterized by infrared spectroscopy and compared with a known caged, proton 2-hydroxyphenyl 1-(2-nitrophenyl)ethyl phosphate (caged HPP). In contrast to caged HPP, caged sulfate can induce large pH jumps and protonate groups that have pK values as low as 2.2. The photolysis mechanism of caged sulfate is analogous to that of P(3)-[1-(2-nitrophenyl)ethyl] ATP (caged ATP), and the photolysis efficiency is similar. The utility of this new caged compound for biological studies was demonstrated by its ability to drive the acid-induced conformational change of metmyoglobin. This transition from the native conformation to a partially unfolded form takes place near pH 4 and was monitored by near-UV absorption spectroscopy.

Excitatory signaling in bacterial probed by caged chemoeffectors

Chemotactic excitation responses to caged ligand photorelease of rapidly swimming bacteria that reverse (Vibrio alginolyticus) or tumble (Escherichia coli and Salmonella typhimurium) have been measured by computer. Mutants were used to assess the effects of abnormal motility behavior upon signal processing times and test feasibility of kinetic analyses of the signaling pathway in intact bacteria. N-1-(2-Nitrophenyl)ethoxycarbonyl-L-serine and 2-hydroxyphenyl 1-(2-nitrophenyl) ethyl phosphate were synthesized. These compounds are a 'caged' serine and a 'caged' proton and on flash photolysis release serine and protons and attractant and repellent ligands, respectively, for Tsr, the serine receptor. The product quantum yield for serine was 0.65 (+/- 0.05) and the rate of serine release was proportional to [H+] near-neutrality with a rate constant of 17 s-1 at pH 7.0 and 21 degrees C. The product quantum yield for protons was calculated to be 0.095 on 308-nm irradiation but 0.29 (+/- 0.02) on 300-350-nm irradiation, with proton release occurring at > 10(5) s-1. The pH jumps produced were estimated using pH indicators, the pH-dependent decay of the chromophoric aci-nitro intermediate and bioassays. Receptor deletion mutants did not respond to photorelease of the caged ligands. Population responses occurred without measurable latency. Response times increased with decreased stimulus strength. Physiological or genetic perturbation of motor rotation bias leading to increased tumbling reduced response sensitivity but did not affect response times. Exceptions were found. A CheR-CheB mutant strain had normal motility, but reduced response. A CheZ mutant had tumbly motility, reduced sensitivity, and increased response time to attractant, but a normal repellent response. These observations are consistent with current ideas that motor interactions with a single parameter, namely phosphorylated CheY protein, dictate motor response to both attractant and repellent stimuli. Inverse motility motor mutants with extreme rotation bias exhibited the greatest reduction in response sensitivity but, nevertheless, had normal attractant response times. This implies that control of CheY phosphate concentration rather than motor reactions limits responses to attractants.

Flash photolysis of caged compounds: new tools for cellular physiology

Caged compounds are molecules or ions of physiological interest, e.g. ATP, IP3, cAMP, cGMP, GTP and Ca2+ rendered inactive by chemical modification. The modification introduces a photochemically labile bond, which on exposure to ultraviolet light cleaves rapidly, releasing the active compound. This article reviews some of the major advances and applications of the photorelease approach, and illustrates its potential in several areas of interest to cellular neuroscientists.